Typically, a radio communication system, which employs TDD (Time Division Duplex) in which transmission and reception are performed in a time-division manner, transmits and receives a signal having the same frequency band over an uplink and a downlink. For this reason, it can be considered that radio channels of the uplink and downlink are equal due to a symmetric property of the radio channels. By using the property, for example, a base station device performs channel estimation of an uplink by using a reference signal of the uplink transmitted from a user terminal, considers a channel estimation value of the uplink as a channel estimation value of a downlink, and performs precoding etc. of a signal of the downlink.
Meanwhile, the radio channels of the uplink and downlink are symmetric and equal, but transmission characteristics of transmitter-receiver circuits of devices such as the base station device and the user terminal are different from each other. For this reason, for example, the channel estimation value of the uplink that the base station device estimates from the reference signal of the uplink does not precisely indicate a channel of the actual downlink.
In other words, for example, the signal of the uplink passes through a transmitter circuit of the user terminal and passes through a receiver circuit of the base station device after passing through the radio channel of the uplink. On the contrary, the signal of the downlink passes through a transmitter circuit of the base station device and passes through a receiver circuit of the user terminal after passing through the radio channel of the downlink. For this reason, the channel of the uplink for which the base station device performs channel estimation includes the transmitter circuit of the user terminal and the receiver circuit of the base station device, and the channel of the actual downlink includes the transmitter circuit of the base station device and the receiver circuit of the user terminal. Herein, because the transmission characteristics of the transmitter circuit and receiver circuit are different between the base station device and the user terminal, the channels of the uplink and downlink are different from each other due to the difference of transmission characteristics of the transmitter-receiver circuits.
Therefore, calibration can be performed to obtain a precise channel estimation value of a downlink from the channel estimation value of an uplink. Calibration is a process for calibrating a difference between transmission characteristics of a plurality of transmitter-receiver circuits. For example, a relative relationship such as phase rotation and amplitude fluctuation in a plurality of channels is obtained by transmitting and receiving a test signal. Specifically, as illustrated in FIG. 9, calibration between a device 1 including a transmitter circuit T1 and a receiver circuit R1 and a device k including a transmitter circuit Tk and a receiver circuit Rk will be explained, for example.
A channel estimation value h1,k and a channel estimation value hk,1 are obtained by transmitting and receiving a test signal between two antennas illustrated in FIG. 9. Herein, the channel estimation value h1,k is an estimation value of a channel that consists of the transmitter circuit T1 expressed with a transfer function T1, a radio channel, and the receiver circuit Rk expressed with a transfer function Rk, and the channel estimation value hk,1 is an estimation value of a channel that consists of the transmitter circuit Tk expressed with a transfer function Tk, a radio channel, and the receiver circuit R1 expressed with a transfer function R1. The ratio of the channel estimation values h1,k and hk,1 is expressed with the following Equation (1).
                                          h                          l              ,              k                                            h                          k              ,              l                                      =                                                            R                k                            ·                              g                                  l                  ,                  k                                            ·                              T                l                                                                    R                l                            ·                              g                                  k                  ,                  l                                            ·                              T                k                                              =                                                    R                k                            /                              T                k                                                                    R                l                            /                              T                l                                                                        (        1        )            
In Equation (1), g1,k and gk,1 are propagation path values of the radio channels, and can be considered as the same value in bidirectional transmission and reception. When a correction coefficient u1 for the transmitter circuit T1 and the receiver circuit R1 of the device 1 is defined as “1” from Equation (1), a correction coefficient uk for the transmitter circuit Tk and the receiver circuit Rk of the device k becomes the following Equation (2).
                              u          k                =                                            u              k                                      u              l                                =                                                                      R                  k                                /                                  T                  k                                                                              R                  l                                /                                  T                  l                                                      =                                          h                                  l                  ,                  k                                                            h                                  k                  ,                  l                                                                                        (        2        )            
By obtaining the correction coefficient uk of the device k in this way, if the correction coefficient uk is multiplied by the channel estimation value hk,1 of the signal transmitted from the device k to the device 1, for example, the precise channel estimation value h1,k of the channel from the device 1 to the device k is obtained.
Calibration is performed also in CoMP (Coordinated Multi-Point transmission) for transmitting a signal to a user terminal through cooperation between a plurality of transmission points, for example. When signals are transmitted from the plurality of transmission points, calibration for calibrating a difference between transmission characteristics of transmitter-receiver circuits of the transmission points is performed. In other words, a test signal is transmitted and received between a user terminal and each of two transmission points to be calibrated, for example, and a channel estimation value of a downlink is fed back from the user terminal, and thus calibration between the two transmission points is performed. As described above, by forming pairs of which each is selected from among the plurality of transmission points and sequentially calibrating each of the pairs, calibration for all the transmission points of the radio communication system can be realized.    Patent Literature 1: International Publication Pamphlet No. WO 2015/022823    Non-Patent Literature 1: Yasunori Nouda, Yoshitaka Hara, Yasuhiro Yano, Hiroshi Kubo, “An Antenna Array Auto-Calibration Method with Bidirectional Channel Measurement for TDD Systems”, technical report RCS 2008-12, IEICE, May, 2008
However, there is a problem that processing amount for forming pairs of transmission points is increased when calibration is performed on a plurality of transmission points. Specifically, as described above for example, when calibration is performed by transmitting and receiving a test signal between a user terminal and each of two transmission points to be calibrated, calibration is difficult to be performed if there is not a user terminal that can wirelessly communicate with two transmission points at the same time. In other words, when an SIR (signal-to-interference ratio) between a user terminal used for calibration and two transmission points does not satisfy a predetermined criterion, for example, reliability of calibration becomes lower even if a channel estimation value of a test signal is fed back from the user terminal. For this reason, when forming pairs of transmission points, a process for determining whether a user terminal satisfying a condition exists is performed with respect to each pair, for example, and thus processing amount forming pairs is increased.
In particular, recently, there is considered a large-scale cooperative system that transmits signals through cooperation between several transmission points connected to one baseband processing device, and thus the conceivable number of pairs of transmission points tends to increase. For this reason, processing amount for determining pairs having a good calibration execution condition is further increased.